The invention concerns a sensor for measuring the difference between the velocity of a cylinder and that of a piston that travels in and out of it at the end of a piston rod in a dashpot.
Such dashpots are employed as intermediates that transmit vibrations from a roadway to the body of the vehicle by way of the wheel suspension. Most of them are variable or regulated hydraulic dashpots, which ensure passenger comfort and maximum passenger safety.
Various parameters of motion, like the difference between the velocity of the body and that of the suspension, or the acceleration of the body, are measured in order to vary the level of vibration suppression exerted by a dashpot in accordance with the condition of the roadway for instance.
German Patent 3 909 190 describes a sensor that can be integrated into a dashpot to measure the difference between the velocity of the body and that of the suspension. This sensor comprises a winding that rests against or is accommodated in the wall of a protective jacket attached to the piston rod and of a permanent magnet in the cylinder's cap, which the rod extends through. When the roadway is irregular, the magnet's flux and its motion within the winding induces a voltage in the winding that is utilized as a signal representing the difference between the velocity of the piston and that of the cylinder and hence between the velocity of the body and that of the suspension.
This sensor, however, has a drawback. Either the jacket that accommodates the winding is made of a non-magnetic material or the winding itself acts as a jacket, wound out of an air coil and extrusion-coated with plastic. Such a winding can intersect only a few of the lines of force extending out of the magnet, and the voltage induced in the winding is not a precise measure of the difference between the velocity of the body and that of the suspension.